Method of insulating conductors



March 11, 1952 c. c. VEALE METHOD OF INSULATING CONDUCTORS Filed April12, 1946 Patented Mar. 11, 1952 METHOD OF INSULATING CONDUCTORS CharlesC. Veale, Wheaton, 111., assignor to Western Electric Company,Incorporated, New York, N. Y., a corporation of New York ApplicationApril 12, 1946, Serial No. 661,577

3 Claims.

This invention relates to insulated conductors and a method ofinsulating them, and more particularly to a conductor encased in avitreous insulating enamel.

In the manufacture of wire wound vitreous en amel covered resistors, alimit on the maximum insulation between adjacent turns is oftenprescribed to permit the winding of compact resistors. These resistorsare often made by winding barewire in uniformly spaced turns around aceramic core after which the whole is covered with an enamel and thenfired to vitrify the enamel coating. However, due to a combination offactors such as a disparity between the relative coefficients ofexpansion of the wire and of the core and the high surface tension ofthe enamel, a condition known in the art as wire pulling takes placecausing adjacent turns to come in contact with each other therebycausing undesirable short circuits between adjacent turns. Shortcircuits are also caused by adjacent turns touching due to slip and playin the wire winding mechanisms. Specifications calling for unusuallyclose winding of turns of fine wire do not permit the use of ordinarywell known wire coverings which are too thick. Any wire insulation usedfor this work must be relatively thin and heat resistant to withstandthe firing temperatures and must tolerate a reasonable amount of fiexureto permit the winding of fine wire on forms of relatively smalldiameter.

It is an object of the invention to provide an easily applied relativelythin heat resistant insulation which will withstand a reasonable amountof fiexure and to provide a method for making the same.

In accordance with one embodiment of this invention aninsulatingcomposition is made by grinding a frit to the fineness ofceramic ink frits and mixing it with a linseed oil base ink varnish as avehicle to form a coating bath. A measured thickness of the compositionis then applied to a wire by passing the wire through the bath andwiping the excess composition off by pulling the wire through a die asit emerges from the bath. The coated wire is then passed directly fromthe bath through a furnace and subjected to a suitable temperature forthe proper length of time to eliminate all organic components of thevarnish and to produce a fully or partially vitrified coating ofcohesive fused particles capable of adhering to the wire.

A complete understanding of the invention will be had by reference tothe following detailed description taken in conjunction with the .of thedie 20.

accompanying single-figure drawing which is illustrative of theinvention.

As shown in the drawing a wire I I, which may be .0025" in diameter, isdrawn from a rotatably mounted supply reel I2 and, after passing over aguide pulley l3, enters a cylindrical tank l4, containing a ceramicenamel slip I5, through a circular die 20, positioned below the liquidlevel in the end wall 2| of the tank. The die 20 which acts as aretainer to prevent the escape of the liquid contents of the tank shouldpreferably be a diamond die having an opening .0031" in diameter. Afterpassing through the ceramic enamel slip t5, the excess enamel is wipedfrom the wire by a circular die 22, preferably a diamond, having anopening about .00315" in diameter. Too large an opening will permit athick coating to be applied to the wire which will cause beading on thewire because the surface tension of the enamel is greater than itsholding qualities. The die 22 is disposed in an end wall 23 of the tankand its opening is in line with the opening A rotatable cylindricalcover 24 encircles the tank and. is provided with an opening to matchthe opening 25 in the top of the tank when rotated to theopen position.

After the coated wire emerges from the tank it is subjected to apredetermined temperature for a specific time period by passing itthrough an electric furnace 30 at a predetermined speed controlled by acapstan 3! around which the wire is passed before being wound on atake-up reel 32 which with the capstan is driven by some correlatedmeans not shown.

The ceramic enamel slip may be prepared from any suitable frit. It hasbeen found that a suitable frit may be made up of the followingingredients in parts by weight as indicated:

Ingredient: Parts by weight Flint 40.41 Red lead 19.07 Borax 30.46Potassium nitrate 5.74 Boric acid 2.87 Chromium oxide .93 Cobalt oxide.52

The various ingredients in powder form are mixed together and themixture is then heated until it melts whereupon it is allowed to flowinto relatively cold water, thus shattering the frit. The frit isfurther ground and pulverized into particles of 6 microns or less indiameter.

Then the frit is mixed with any of the ink varnishes well known in thetrade and which have a base composition of boiled linseed oil. Thedispersion of the frit must be thorough so as to result in a homogeneousmixture. Such a mixture is known in the art as ceramic ink.

When hard, smooth wire is being coated, the enamel slip will "bead onthe wire if the liquid is too thin. The consistency of the ceramicenamel slip may be controlled by the blending of a number of inkvarnishes having different grades of viscosity.

The speed with which the wire is to be drawn through the furnace,depends upon the temperature and length of the furnace. The temperatureand the time interval during which the coated wire is subjected to thattemperature should be correlated so as to eliminate all the organiccomponents in the coating and produce a fully or partially vitrifiedcoating of cohesive, fused particles adhering to the wire.

In the herein described embodiment of the invention, the length of thefurnace is 12 and the following temperature and wire travel speedcombinations were found to be successful: 103" per minute at a furnacetemperature of l440- 1460 F., 323" per minute at a furnace temperatureof 1500 F. One may choose any combina tion of time and temperature thatwill drive off all the organic material from the ceramic enamel andproduce the desired degree of cohesive fusion between the frit particlesin the enamel and efiect the necessary adherence to the wire.

It is not known whether this method provides a, fully vitrified orpartially vitrified coating. However, it does produce an enamel coatingthe particles of which are held together 'by at least partial fusion.

An important advantage of this coating material and method for applyingthe same is that very thin wire may be covered with a thin, relativelyflexible insulation thus permittingv the winding of small diameterresistors or other electrical parts having a minimum of insulationbetween adjacent turns which will not be short circuited in case theturns touch because of wire pulling or other causes.

What is claimed is:

1. A method of insulating. a conductor comprising applying to saidconductor a ceramic enamel containing a frit reduced to particles 6microns or less in diameter mixed with a boiled linseed oil base inkvarnish, and heating said coated conductor to such a temperature aswould be provided by moving said article through a 12" furnace within arange of speed and temperature in the furnace from 103 inches per minuteand 1440 F. to 325 inches per minute and 1500 F.

2. A method of insulating a conductor comprising applying to saidconductor a ceramic enamel containing a frit reduced to particles 6microns or less in diameter mixed with a boiled linseed oil baseinkvarnish, and heating said 15 coated conductor to such a, temperatureas would be provided by moving said article through a 12" furnace at aspeed of 103" per minute, the temperature of said furnace being 1440 F.

3. A- method of insulating a conductor comprising applying to saidconductor a ceramic enamel containing a frit reduced to particles 6microns or less in diameter mixed with a boiled linseed oil base inkvarnish, and heating said coated conductor to such a temperature aswould be provided by moving said article through a 12" furnace at a,speed of 325" per minute, the temperature of said furnace being 1500 F.

CHARLES C. VEALE.

REFERENCES CITED The following references are of record inthe file ofthis patent:

UNITED STATES PATENTS Iron, Technologic Papers of the Bureau ofStandards (part of vol. 17), December 19, 1923.

